1999 VW Cabrio AHU TDI Swap/Conversion

So, I figure it was finally time for me to start a thread on my project. My work on it has been slowing down and I was hoping doing this would inspire me to get back to it and get 'er done! Much thanks goes out to Lug_Nut and Uberhare who have provided me with guidance before I started on this endeavour and many thanks in advance to them and everyone else who will help me to get this project finished.
The donor for this project was P2B's beloved 1998 AHU TDI with 248,000km. It had suffered some front end damage and upon further inspection, the driver's side strut tower and rocker panel was found to be rusting and not worth the cost of repair. My dream to have a TDI Cabrio was just a dream at first until P2B messaged me one day saying he had the perfect donor for me.









Having found a full donor vehicle, my next step was to find a suitable recipient. I found it extremely difficult to find a MK3 with little to no rust, and most Cabrios in good shape were way out of my budget. I originally wanted a white or champagne coloured one but later relaxed my expectations. I ended up getting a red 1999 Cabrio with AT, leather interior and power top (although non-functioning).

Unfortunately I didn't get any shots of the car before the whole thing began.


2.slow engine being prepped to be removed from Cabrio


AHU being prepped to be removed from Jetta


2.0 and subframe being lowered onto tires


Cabrio engine bay empty...ready for something better!


AHU and subframe pulled

Cost to date: $750 1998 donor TDI + $2000 1999 Cabrio = $2750
Time spent to date: ~8 hrs x 4 guys = 32 hrs

AHU in all its glory




AHU in its new home!




Gasoline tank pulled...comes with free gas!


Anyone need a 2.0 with auto tranny?


Interior of donor being stripped of needed parts.


Shifter boxes - auto on top and manual on bottom


Shifter boxes - auto on left and manual on right


Shifter cable ends

Cost to date: $2750 (unchanged)
Time spent to date: 32 hrs + 5 hrs = 37 hrs

I worked a bit more on the donor yesterday and finally got the AHU engine harness out.


On the donor car, I basically unplugged all the connectors on the fuse box and pulled the ones from the engine harness through the firewall. On the recipient, I have been trying not to do this and only pulling the ones that are from the engine harness. Its taking quite a bit longer because I have to trace the wires to the appropriate plug and do this through the birdnest of wires. If I did this, could I merge the donor AHU harness onto the recipient fusebox? I'm assuming that there would also be some additional custom wiring required for slight differences in the cluster (ie., glowplug light?)


Engine harness interior side

I was trying to remove the pedal cluster but had a lot of trouble with the cross brace and steering column in the way so I decided to take it out. Is this really needed? I didn't mind doing it on the donor car but am hoping I won't have to do it on the recipient car. According to my Bentley, a special VW tool is required to remove the brake booster arm ball from the brake pedal's socket?





Any help is much appreciated!


Cost to date: $2750 (unchanged)
Time spent to date: 37 hrs + 3 hrs = 40 hrs

Did a little more work today. Got the rest of the engine wiring harness plugs out of the fuse box in the Cabrio and pulled them through the firewall. Now I'm gonna compare the two harnesses and see what the major differences are and if I can plug the AHU hardness back into the same places on the fuse box.

Also got the gasoline evap system out of the Cabrio to save some pounds. While I was taking the fender liner out of the passenger side front, I found a sandbox at the bottom of the fender. Looks like the fender will have to be replaced eventually...when I get the car painted I guess.


Cost to date: $2750 (unchanged)
Time spent to date: 40 hrs + 4 hrs = 44 hrs

AHU harness should plug right in fine. There is 1 mystery wire on a 4 pin yellow connector (that I have yet to trace) that will cause your turn signal relay to blow when signalling right. Some difference between Cabrio and Jetta, mine's unplugged - everything working so far. I don't think it's even documented in the Bentley either. Lugnut came across this as well.

At least your car is a late one with the larger hole through the firewall. My was a 95 where the light harness went through a seperate hole. What a PITA.

As I offered before, you're welcome to check out my car anytime if you get stuck.

Required? No. I don't have one and I managed to get it out.
It basically entails pulling the rod out of the white clip, (the pedal arm spreads enough to allow this) then compress the sides of the clip (no ball in the way so it is possible, not easy, but possible) and with your third hand pry the clip out.
Place the clip on the new rod and use the rod to press the clip into the pedal arm.

Earlier you wrote: "If I did this, could I merge the donor AHU harness onto the recipient fusebox? I'm assuming that there would also be some additional custom wiring required for slight differences in the cluster (ie., glowplug light?)" Will you be keeping the Jetta TDI cluster for the Cabrio, or using the gas cluster? My Cabrio was clustered by the prior owner so the title was already branded "True Mileage Unknown". I didn't have an incentive to keep the odometer number sequence so out went the gas cluster. My Jetta donor had almost precisely 100k miles more than the discarded gas cluster so it's easy to keep track.

I can put you in contact with someone to reprogram the cluster. It is a serial eeprom on the board. Or if you're any good with a soldering iron, swap the chip...... If you look there is even a 4 pin plug on the back of the cluster for a programming cable to be plugged in.

I'm planning to swap the TDI cluster over. There's not reason not to do so. Since I have a full donor vehicle, I plan to use as much of it as I can. I noticed the cluster plugs for both vehicles are the same. If I swap the TDI cluster into my Cabrio, will it have full functionality or will I need to do some custom running of wires to get everything working (like I have read in other threads). I also have the cluster wiring harness from the TDI out of the donor, since all the plugs were completely removed from the fuse box.

Thanks for the tips!

The pedal clip to which I made reference is actually the CLUTCH pedal connection to the clutch master, not the brake. I don't recall any brake arm issues.
Swap the full cluster and the blue / red connectors and their white small gauge wires with it. There will be no need to replace or fabricate any wiring.
The Cabrio's current air bag (and something else, too) warning lamp panel to the left of the headlight switch will be redundant. Its function will be replaced by lamps already in the Jetta cluster. Either swap in the Jetta's blank dummy panel or leave the Cabrio's warning lamp panel in position, but now without input connection.

Here are some more pictures of things removed today.


This was part of the evap system for the gasoline. What is the big fender shaped thing? I figure it's some sort of air dryer.


Took the throttle cable and coil out. Can someone tell me what's supposed to mount in the spot of the TDI where the coil would have mounted in the gasser? I took a look in my donor and whatever was there was already removed and I can't seem to figure out what goes there.

I was in the process of removing the auto pedal cluster from the gasser and traced some vacuum lines to this:


When I pulled it out, it was this...


...and there was a harness connecting it to this module inside the cabin:




I wasn't sure what it was at first but I figured it may be the cruise control stuff since there were vacuum lines going to both the accel and brake pedals. After some investigation, this was confirmed. My TDI donor did not have this module so I guess it didn't have cruise. Did manual transmission cars ever come with cruise?

Here are both cross braces from the automatic Cabrio and manual TDI donor:


The Cabrio ignition swtich has the auto tranny shiftlock cable. As long as its detached and I can turn the key, I'm good right? I don't need to swap the ignition switches over?

Also noticed the alarm/keyless entry modules on the two car were different:


Module in 1999 Cabrio with part number 1HM 937 045 P


Module in 1998 Jetta TDI with part number 1HM 937 045 Q

I'm wondering if I should swap the TDI module over to the Cabrio one too. When I bought the Cabrio, the driver's door lock would lock the door mechanically with one turn but would not arm the alarm (with the two turn method). I had to use the passenger side to do that. I figured that was just a problem at the door's end and possibly a loose wire. Could it be a faulty module that would allow one door lock to work and not the other? If I swap the TDI one over, will it work with the Cabrio? I guess there will be a lot of testing/troubleshooting once more things are back together.

I'm having a heck of a time trying to get the pedal clusters out of both cars. I gave up on the TDI one for now because I thought I would try on the automatic one first since it would be okay if I accidentally broke something. I'm having trouble getting the brake piston ball out of the socket in the brake pedal because I can't see behind it. Do I just unbolt all the bolts holding the cluster to the firewall and brake booster and pull? Or is there a trick?

Cost to date: $2750 (unchanged)
Time spent to date: 44 hrs + 1.5 hrs = 45.5 hrs

Looks like I may have to take a hiatus from the TDI project for a bit. Just noticed a leak of sorts under my MK1 Cabriolet. My landlord is getting on my case about it so I'll have to try to figure that out asap before I can get back on the TDI.

Many of these have been covered in previous TDI into Cabrio swap threads.

Keep the engine operating stuff with the engine.
Cruise control is through the ECU in the TDI. Scrap all the 2.0 cruise crap,
but not just yet. Wait until the swap is complete and fully functional.

Keep the body operating stuff with the body.
The alarm modules are different because the number of door switches is different. KEEP the Cabrio alarm module.

The ignition key is your call. Do you want two keys, one for the Cabrio doors, another for the Jetta ignition lock? Me? I'm lazy I'd defeat the cable and keep the Cabrio lock.

Thanks Lug Nut. Now that you mention it, I do remember reading about all of that stuff in your and Uberhare's thread.

Great thread! Best of luck with your conversion!

Did a little bit of work today, cleaning up the battery tray and taking apart the rad fan shroud assembly to repaint. Picked up two cans of Tremclad for the job.





I tried to give the pedal cluster in the Cabrio a closer look but I forgot to bring a mirror to the shop. I've read different ways to remove it such as using a dremel to cut the brake booster shaft or just giving it a good yank. Because I'm really trying to salvage the parts and not break anything, I've been pretty cautious. The special VW tool that is needed (according to the Bentley) looks like this but I'll be trying to make something that does the same thing.



Cost to date: $2750 + $12.50 spray paint = $2762.50
Time spent to date: 45.5 hrs + 1.5 hrs = 47 hrs

I finally got some time again to do some work on the swap and managed to pull the pedal clusters out. I gave them a good yank and was able to pull them out without damaging much (as it appears so far).

Here's the auto pedal cluster from the Cabrio:


...and the manual pedal cluster from the TDI:


The throttle position sensor:


and the bracket for the 02A clutch master cylinder:


For reference, here is what the back of the brake pedal looks like, where the brake booster rod ball inserts into:


...and the back of the clutch pedal:


Cost to date: $2750 + $12.50 spray paint = $2762.50
Time spent to date: 47 hrs + 0.5 hrs = 47.5 hrs

I recently dislocated a finger and got really busy with work and life so the project has been on the back burner.After about a month of not working on the car, I finally got back on it this past weekend and surprisingly made some progress. Its amazing how much you can accomplish when you're actually focused.

I got the wiring harness in one day, plugging in as much as I could on the cabin side and on the engine side. Unfortunately, when I friend helped me pull the engine harness from the TDI donor, we hadn't labeled all the connectors. So now I'm left with a bunch of connectors, a Bentley, and a lot of work ahead.

My friend came the next day to help with the investigating and we were able to figure out a bunch of the wiring. The following are pictures of things that are still mysteries. Maybe someone can help.


This harness came through a hole in the firewall separate from the main engine harness. Tracing it back to inside the cabin, part of it goes along the side of the car. I believe this is the auto tranny harness that goes to the Transmission Control Module under the rear seats. Can I pull all of this if I've converted to an manual 02A?

On the inside of the cabin:






How do I mount this fuel stuff in the engine compartment?




If anyone can post a picture of the strut tower areas of their AHU engine bay, that would be much appreciated.

Also, how do I move the accessory belt tensioner in order to get the belt back on?


While trying to install the clutch slave cylinder back into the tranny, I managed to tear a seal. I got a new clutch slave now but haven't put it in yet. Is there a way to install it? When I look into the hole where the clutch slave mounts into, it seems pretty solid and doesn't have a hole or anything for the shaft of the clutch slave. Am I missing something?

So after getting most of the wiring sorted out, we constantly tried starting the car to see if it would at least crank. Scanning with VAG-COM yielded a bunch of codes that made no sense. Finally, it occurred to me that in the past every time i connected a battery, the alarm would sound because there was a problem with the door locks but we never heard anything. This was when I realized that I had disconnected the horn so I connected it back so that I would at least know if the horn was going off. When I did this, it totally dawned on me that the alarm/remote door module must need to be connected in order to start the car (as a security feature). On connecting the module, the car started on the first try!


AHU Cabrio TDI first start

I then cleared the codes with VAG-COM and scanned the car again and came up with these codes:

VAG-COM Version: Release 409.1-S

Control Module Part Number: 028 906 021 JB
Component and/or Version: 1,9l R4 EDC G10SG 1457
Software Coding: 00002
Work Shop Code: WSC 00066
3 Faults Found:
00626 - Glow Plug Indicator Light (K29)
31-10 - Open or Short to Ground - Intermittent
00741 - Brake Pedal Monitoring
27-00 - Implausible Signal
00750 - Warning Lamp
31-10 - Open or Short to Ground - Intermittent

Luckily, nothing seems to be critical so I guess I'll just have to do a bit more investigating to get the wiring all sorted out.


Cost to date: $2762.5 + $62.50 clutch slave = $2825
Time spent to date: 47.5 hrs + 12.5 = 60 hrs

Been there, done that.
Look through the build thread I put together for mine. Some of your questions I had myself. You can see how I addressed them.

Which wiring harness did you use? You should have used the TDI harness. That top image with the cylindrical plug is an unknown to me. My manual to manual didn't have it. Your TDI harness shouldn't either.

Grab the Bentley. Look through all the pages for a description of T1 and T2 (terminations with one or with two conductors) and having color codes that match the wires shown in image two.

The circuit breaker in image 3 is for the power windows.

The single connectors in image 3 and 4 look like "TV" connectors that are all plugged into a four way bridge module (plug-in substitute for a splice) of matching colors. Just be certain that at least one of those four is powered to supply to the others in that module.

The EGR control solenoid/valve mounts under the fuel filter bracket.

The N75 valve mount bracket has to be removed from the TDI donor and transplanted into the Cabrio. Mount holes exist already. I used blind rivets for mine.

c5ho said:

Also, how do I move the accessory belt tensioner in order to get the belt back on?

Click to expand...

Seriously?!?! You have sufficient confidence in your abilities to do an engine swap, you have a Bentley service manual, yet you are unaware of how to re-mount the ribbed belt?

The alarm issue took me a while to figure out, too. Basically the entire TDI harness and alarm module has to be used. I really hope that you still have the TDI donor available from which to claim the needed items.

Thanks Lug-Nut. I re-read your build thread last night too to get more information that I had forgotten. I used the TDI harness on the engine side and pretty much kept everything stock on the cabin side of the Cabrio.

It was a couple of months ago when I took the belt of the tensioner and for some reason I just couldn't get it to go back on the other day.

Did a bit more work today. Got the rad fans assembled back into the shroud and the shroud mounted back onto the rad. Also got the serpentine belt back on the accessories and tensioner. The spring in the tensioner arm felt a bit weak and that was why I was having trouble moving it out of the way. Every time I would crank the tensioner to rotate it out of the way, the tensioner bolt would just tighten.

Lots of wiring still left to sort out. Noticed some problems with the door locks today so I may fix that problem first. I'm planning to get the engine bay more or less buttomed up first so that it at least looks like a car again. Tomorrow, I plan to install the rad support, rad, cooling, clean up the wiring harness, mount the ECU. Lots planned with a long day ahead!

Cost to date: $2825
Time spent to date: 60 hrs + 2 hrs = 62 hrs

The car is pretty much together now in terms of engine compartment. From the outside, it looks like a car. The only thing left now is the interior. Today I worked on getting the dash in. I had to do it a second time because I forgot to plug in the speakers the first time. Can anyone help me figure out what this connector is for?



Its a small blue connector. I can't seem to figure out where it goes in the dash. Also, my turn signal indicators aren't working in the cluster. The turn signal lights work but when they are flashing, the cluster indicators are blank. Also, brake lights are always on so I unplugged the brake light switch for now. I figured there would have to be some electrical troubleshooting. Tomorrow I will fill all the fluids and give her a go!

Thanks in advance.

That, I believe, is a connector for the clutch pedal cruise control interrupt.
The instrument cluster turn signals might not be plugged in yet. Look for both of the two conductor plug sets, one wire is black with white, the other is black with green. There are two of these plug sets: T2n, and T2l, both "n" and "l" need to be plugged in to their own mate.

Thanks Lug_Nut! I'll look up the wiring diagrams but you've definitely given me a head start!

http://forums.tdiclub.com/showthread.php?t=249433&page=2
post #45,
That's the same connector set you need to find, but I don't know if you'll have the same 3 or 2 plugs. There will be two wires, but there might be an unused third socket position available.